The present invention relates to an aqueous ink composition suitable for use as printing ink. More particularly, it relates to an aqueous ink composition for ink-jet recording, which is capable of yielding printed images with high quality (i.e., high clearness and sharpness, etc.), and which is also capable of providing stable ink ejection, without causing the plugging of the nozzles of the ink-jet recording apparatus, even if it is used continuously for a long period of time or if it is used intermittently after a period of non-use of the ink-jet recording apparatus.
Generally, an aqueous ink comprises as the main components a dye, water and a humectant. In particular, an aqueous ink for use in ink-jet recording should satisfy the following conditions in order to yield an excellent print:
First, in order that the ink be compatible with ink droplet formation and control of the direction of the ejected ink droplet streams, it is necessary that its viscosity, surface tension, specific electric conductivity, and density each fall within certain appropriate ranges.
Second, it is necessary that no precipitates be separated, due to coagulation, precipitation, chemical change of slightly soluble components or other causes, from the ink during an extended period of continuous use or storage, or during the periods when the apparatus is not in use. Nor should the physical properties of the ink otherwise change during the above-mentioned periods. If the separated solid components or viscous materials in the ink become affixed around the nozzles, or if the physical properties of the ink change, deviating from the predetermined physical properties achieved at the time of the preparation, the desired printing quality, the desired ink ejection stability, and the desired ink ejection response cannot be obtained.
Third, it is desired that the ink provide adequately high contrast, clearness and the desired color in the printed image, in particular, for use with full-color printers.
Fourth, it is necessary that the ink for ink-jet recording be such that the printed images are resistant to water, light and physical wear.
Fifth, it is necessary that the images printed by the ink dry rapidly.
So far, many proposals have been made for an ink for ink-jet recording, but from the viewpoint of practical use, a satisfactory ink which is capable of meeting the above-described requirements has not been found.